Context for project index operations. There can be only one such operation
per project at any given time.
This factory method is called to build a progress URL so the TaskDescriptor can allow some one to navigate to the
task's "web page". The URL should start with "/" and be ready for the servlet context path to be prepended.
||- the id of the task. Since this is not known until task submission, this call back is informed of it once created.
- returns a context specific progress URL that a user can go to to view the progress of a task.